There are many applications in which it is desirable to energize a three-phase A.C. induction motor from a single-phase power line, which may be the only available source of A.C. power at a given location. A conventional technique, in these circumstances, is to connect two of the three stator winding power input terminals directly to the power line conductors and to couple the third stator winding terminal to one of the power line conductors through a dephasing capacitor. For starting, an additional starting capacitor is connected in parallel with the dephasing capacitor. With this simple arrangement, the three-phase motor functions in a manner analogous to a capacitor-start capacitor-run single-phase induction motor.
This conventional static phase conversion circuit for operation of a three-phase motor from a single-phase A.C. line exhibits rather low efficiency and has a relatively poor power factor at full load, even with careful selection of the dephasing capacitor. For a motor run at variable loads or even at a relatively constant load, the power factor and efficiency are never really satisfactory.